Like the previous poster said, everyone is different but I can share my experience. I had a total knee replacement on one knee on Oct 30th of last year. We were given a free trip in June so my trip was almost 8 months out. I think it was a little too soon but who could resist a free trip. I did not rent an
ECV because I thought that I could just take it slow and rest a lot. (I also did not trust my ability to drive one.) The biggest problem is that WDW just does not have that many benches or other places to sit. After an hour or so of walking around I needed to rest my leg and it was sometimes hard to find a place. By nighttime everyone in the parks seemed to be tired and the few benches we're always taken.
It may seem contradictory but I did have a good time. My leg just wore out. And I have been almost compulsive about exercising. I work out with weights and cardio five days a week and have been doing this for three years. It just takes time to heal.
As far as rides, I was able to do everything, even the roller coasters. I worried about being able to get in and out of rides like Space Mountain or Rock and Roller Coaster but didn't really have a problem. I just held onto the ride vehicles and lowered myself or pulled myself out. I do have two big cautions. Be very careful on Test Track. The step down into the car was much steeper than I expected and I really hurt my new joint getting in. I think this may have been responsible for a lot of my trouble for the rest of the trip. The only other ride that gave me trouble was Star Tours. I was seated in the back row and my feet were too far off the floor (I don't know if the other rows are lower). The ride really slings you around and with nothing to brace your leg on my knee hurt during that ride.
I'll be glad to answer any other questions.